Implantable medical devices that are minimally invasive and self-contained are increasingly popular as modes for improving medical care. For example, wireless sensors, leadless stimulators, and drug delivery pumps are some implantable medical devices that are becoming pervasive tools used in medical treatment. Some of these devices are powered by batteries and may operate continuously until the battery is depleted or may intermittently operate with spaced apart “sleep” intervals so as to extend battery longevity.
In designing implantable medical devices, a trade-off is commonly encountered between competing needs for a small device size and for a battery with sufficient capacity to meet longevity goals. The battery is oftentimes the largest component in the implanted medical device and therefore significantly impacts the size of the implantable medical device.
To minimize battery size, it is generally desirable to design a device that draws a negligible amount of current until a deterministic activation occurs. Such design should also be relatively immune to inappropriate activations so as to avoid unnecessary current consumption.
Conventional reed switches have been used in the past with implantable medical devices. For example, a conventional reed switch has been used between the battery and the device circuitry of a pH sensor. As packaged, the reed switch is biased open (i.e., the device is “turned-off”) by an external biasing magnet placed over the sensor during manufacture. The pH sensor is “turned-on” just prior to implantation by removing the external biasing magnet.
However, this conventional reed switch generally tends to be too large in size and thus adds significant size to the implant device. Additionally, the reed switch may be susceptible to environmental effects (e.g., magnetic or mechanical) that may result in inappropriate sensor activation. Smaller conventional reed switches, such as micro-electrical mechanical systems (MEMS) reed switches, may not pass sufficient current and are generally not suitable as a drop-in replacement for the conventional reed switch, such as previously described, where higher currents are used. Additionally, MEMS reed switches are generally susceptible to the same environmental effects that may cause inappropriate activation in conventional reed switches.
Other implantable device designs have used monolithic Hall effect sensors, radio frequency (RF) signaling, and ultrasound to activate the device. Monolithic Hall effect sensors have been used in the past to change the state of a device based on a sensed magnetic event. Hall effect sensors typically require a stand-by current and are generally unsuitable as a means for turning a battery-powered device on or off. In implementations utilizing RF signaling, the implanted device “listens” continuously for a unique RF signal. RF signaling shares a similar current consumption characteristic with Hall effect sensors by generally requiring some current to be used while the device is in a stand-by mode. An ultrasound transducer has been used to turn on a switch that subsequently powers a wireless sensor containing a very small battery. In this approach, the ultrasound transducer should be in effective contact with a patient's body to appropriately activate the sensor.
Accordingly, it is desirable to provide an activation device for implantable medical devices that has relatively low or negligible current draw until a deterministic activation occurs. It is also desirable to provide an activation device for implantable medical devices that gates current to the devices while avoiding significant capacity drop to a shared power source and that does not significantly contribute to an overall size of the device. In addition, it is desirable to provide an implantable and minimally invasive medical device having reduced susceptibility to environmental effects that may result in inappropriate activation thereof. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.